In 2010, I was fortunate to be involved in an exciting piece of research led by Matt McGlone at Landcare Research in New Zealand. Published in Nature Geoscience, we reported a long-term record of vegetation and climate change on a sub-Antarctic island known as Campbell. Located at the far southern end of the Pacific, Campbell Island sits at an impressive 52˚S, entirely surrounded by ocean. By looking back through the ancient peat sediments on the island, we found large changes in the different types of preserved pollen grains over the past 18,000 years. It all pointed to big swings in vegetation across the island. Fortunately, because the vegetation is very sensitive to summer conditions, we were able to get a handle on temperatures in the past. Fascinatingly, the island temperatures didn’t appear to track those recorded by ocean cores from the region.
|King penguin, South Georgia|
Previous work on temperature records from this part of the Southern Ocean had consistently shown warming began at the end of the last ice age around 18,000 years ago, peaked at warmer than present day temperatures (up to 3˚C warmer) between 12,000 and 8000 years ago and thereafter cooled. In contrast, the Campbell Island results showed summers remained cooler than now until around 9000 years ago, after which they warmed to present day levels. The most likeliest explanation for this divergence of the ocean and land temperature trends is the changing position and intensity of the westerly winds. Today, the strongest westerly winds in the Southern Ocean lie directly over the island. It looks like the vast north-south interchange of heat caused by these westerly winds was established at their current latitude some 9000 years ago, drawing warm air south over the island in summer (relative to the ocean) but having less of an effect in winter, leading to cooling.
The Campbell Island record of climate points towards large changes in the position and intensity of the westerly winds in the past. The winds appear to be very sensitive to subtle changes in climate across the southern hemisphere which in turn may have had a major influence globally. To test these ideas, a PhD student, Alan Williams, and myself, are visiting South Georgia, an island in the Atlantic sector of the Southern Ocean, as part of funding provided by the Australian Research Council. South Georgia is ideally placed to investigate past vegetation and climate changes. The island is some 170 km long and lies immediately downwind of the Drake Passage, through which the Antarctic Circumpolar Current (sometimes shortened to ACC) is forced while travelling from west to east around Antarctica. Importantly, the ACC is a current driven by the Southern Hemisphere westerlies. As a result, the climate of today’s South Georgia is dominated by frequent low pressure weather systems that cross the island, bringing large amounts of rain (on average, a metre and a half each year). If anywhere should pick up changes in westerly airflow in the southern part of the Atlantic, South Georgia should preserve a record of it.
As part of our fieldwork, Alan and I will be looking at peat and lake sediments that span the last 2000 years, using similar methods to Campbell Island, to try to get a precise record of what the climate did on decadal and centennial timescales. Alongside this work, we’ll be taking samples from moraines, ramparts of rock marking the former extent of glaciers, for Dr Chris Fogwill to date their rate of retreat and compare to the climate reconstructions made from the island and those we collected recently from Antarctica. It should give us some real insights into how these high latitude environments respond to past and future climate change. Unfortunately, the downside is a several day boat trip from Port Stanley on the Falkland Islands (each way). Oh well.
Just as with our Antarctic expedition, you can follow our fieldwork in South Georgia (and the crossing) courtesy of Twitter by subscribing to ProfChrisTurney. It’s going to be a long journey but with any luck the trip should be well worth it.